N-Secondary butyl glycine promoted acid gas scrubbing process

ABSTRACT

The present invention relates to an alkaline salt promoter system which includes N-secondary butyl glycine and its use in acid gas scrubbing processes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the use of N-secondary butyl glycine asa promoter for alkaline salts in "hot pot" type acid gas scrubbingprocesses.

2. Description of Related Patents

Recently, it was shown in U.S. Pat. No. 4,112,050 that stericallyhindered amines are superior to diethanolamine (DEA) as promoters foralkaline salts in the "hot pot" acid gas scrubbing process. U.S. Pat.No. 4,094,957 describes an improvement to this process whereby aminoacids, especially sterically hindered amino acids, serve to preventphase separation of the aqueous solution containing sterically hinderedamines at high temperatures and low fractional conversions during theacid gas scrubbing process.

One of the preferred sterically hindered amines described in thesepatents is N-cyclohexyl 1,3-propanediamine. The bulky cyclohexane ringon this diamino compound provides steric hindrance to the carbamateformed at this site thereby favoring the expulsion of CO₂ duringregeneration, thereby leaving the hindered amine group free toprotonate. The primary amino group of this diamino compound assists inmaintaining solubility under lean conditions. Under lean conditions whenthere is insufficient carbonic acid present to protonate the hinderedamino group, the molecule would be insoluble were it not for the primaryamino group which forms a stable polar carbamate ion. However, even thecarbamated primary amino group is insufficient to prevent insolubilityof the compound under very lean conditions and an additional additive,as proposed in U.S. Pat. No. 4,094,957, an amino acid, is required tomaintain solubility of the diamino compound. This amino acid alsocontributes to additional capacity and faster absorption rates forcarbon dioxide, so it therefore acts as a copromoter in addition tosolubilizing the sterically hindered diamino compound. Screening studiesof available amino acids as possible copromoters for N-cyclohexyl1,3-propanediamine based on cyclic capacity and rates of absorptionascertained that pipecolinic acid was one of the best amino acidcopromoters.

Subsequent studies, however, have demonstrated that theN-cyclohexyl-1,3-propanediamine-pipecolinic acid promoter system hasseveral shortcomings. Firstly, N-cyclohexyl-1,3-propanediamine is bothchemically unstable and volatile. For example, it degrades into a cyclicurea in the presence of hydrogen sulfide. In fact, the rate of cyclicurea formation has been found to be highly dependent on hydrogen sulfideconcentration, a common contaminant of industrial acid gas streams. Thecyclic urea formation from this diamine is favored by the stability ofthe six-membered ring structure of the cyclic urea. In addition topromoter losses due to cyclic urea formation, which may be a seriousproblem with hydrogen sulfide rich streams, the cyclic urea product haslimited solubility, and its separation from solution poses additionalproblems. Various techniques for coping with this water insoluble cyclicurea have been proposed. See, for example, U.S. Pat. Nos. 4,180,548 and4,183,903. However, these techniques have specific benefits andproblems, e.g., specialized equipment is necessary.

Pipecolinic acid also has shortcomings, e.g., it is rather expensive andits picoline precursor is in limited supply.

In view of the commercial potential of using the sterically hinderedamino compounds as described and claimed in U.S. Pat. Nos. 4,094,957 and4,112,050, there is a need for finding sterically hindered aminocompounds which perform as well as N-cyclohexyl-1,3-propanediamine butdo not have the volatility and degradation problems of this compound.Also, there is a need for finding a less costly replacement forpipecolinic acid which possesses its effectiveness. Preferably, there isa need for finding a single amino compound which performs as well ornearly as well as the N-cyclohexyl-1,3-propane-diamine/pipecolinic acidmixture, but not suffer the preparative cost volatility and degradationproblems of this mixture. Such a discovery would be of significanttechnical and economic merit.

Various amino acids have been proposed as promoters for alkaline saltsin the "hot pot" gas scrubbing process. For example, British Pat. No.1,305,718 describes the use of beta and gamma amino acids as promotersfor alkaline salts in the "hot pot" acid gas treating process. Theseamino acids, however, are not suitable because the beta-amino acidsundergo deamination when heated in aqueous potassium carbonatesolutions. The gamma amino acids form insoluble lactams under the sameconditions. Also, the alpha-amino acid, N-cyclohexyl glycine, asdescribed in Belgian Pat. No. 767,105, forms an insolublediketopiperazine when heated in aqueous solutions containing potassiumcarbonate.

SUMMARY OF THE INVENTION

It has now been discovered that N-secondary butyl glycine, a stericallyhindered monosubstituted alpha-amino acid, is an excellent promoter foralkaline salts in the "hot pot" acid gas scrubbing process. This aminoacid, when used as the sole promoter, not only provides for high carbondioxide capacity and high rates of carbon dioxide absorption, but doesnot form undesirable insoluble degradation products as in the case ofN-cyclohexyl-1,3-propanediamine, the beta and gamma amino acids and thealpha amino acid, N-cyclohexyl glycine. Also, this amino acid is lessvolatile than N-cyclohexyl-1,3-propanediamine, thereby the economy ofthis promoter is greater than the previously employed promoters. Inaddition, this amino acid is superior in terms of carbon dioxidecapacity and rates of absorption for carbon dioxide than pipecolinicacid and related amino acids. Furthermore, N-secondary butyl glycine canbe prepared from relatively inexpensive compounds thereby reducing thecost compared to the use of pipecolinic acid.

Accordingly, in one embodiment of the present invention, there isprovided a process for the removal of CO₂ from a gaseous streamcontaining CO₂ which comprises contacting said gaseous stream (1) in anabsorption step with an aqueous absorbing solution comprising (a) abasic alkali metal salt or hydroxide separated from the group consistingof alkali metal bicarbonates, carbonates, hydroxides, borates,phosphates and their mixtures, and (b) an activator or promoter systemfor said basic alkali metal salt or hydroxide comprising at least aneffective amount of N-secondary butyl glycine; and (2) in a desorptionand regeneration step, desorbing at least a portion of the absorbed CO₂from said absorbing solution.

As another embodiment of the present invention, there is provided anacid gas scrubbing composition comprising: (a) 10 to about 40% by weightof an alkali metal salt or hydroxide, (b) 2 to about 20% by weight ofN-secondary butyl glycine, and (c) the balance, water.

In general, the aqueous scrubbing solution will comprise an alkalinematerial comprising a basic alkali metal salt or alkali metal hydroxideselected from Group IA of the Periodic Table of Elements. Morepreferably, the aqueous scrubbing solution comprises potassium or sodiumborate, carbonate, hydroxide, phosphate or bicarbonate. Most preferably,the alkaline material is potassium carbonate.

The alkaline material comprising the basic alkali metal or salt oralkali metal hydroxide may be present in the scrubbing solution in therange from about 10% to about 40% by weight, preferably from 20% toabout 35% by weight. The actual amount of alkaline material chosen willbe such that the alkaline material and the amino acid activator orpromoter system remain in solution throughout the entire cycle ofabsorption of CO₂ from the gas stream and desorption of CO₂ from thesolution in the regeneration step. Likewise, the amount and mole ratioof the amino acids is maintained such that they remain in solution as asingle phase throughout the absoption and regeneration steps. Typically,these criteria are met by including from about 2 to about 20% by weightof preferably from 5 to 15% by weight, more preferably, 5 to 10% byweight of this sterically hindered monosubstituted amino acid,N-secondary butyl glycine.

The aqueous scrubbing solution may include a variety of additivestypically used in acid gas scrubbing processes, e.g., antifoamingagents, antioxidants, corrosion inhibitors and the like. The amount ofthese additives will typically be in the range that they are effective,i.e., an effective amount.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 graphically illustrates the vapor-liquid equilibrium isothermsfor potassium carbonate solutions activated by equal nitrogen contentsof N-secondary-butyl glycine and diethanola mine at 250° F. (121.1° C.)wherein the CO₂ partial pressure is a function of the carbonateconversion.

DESCRIPTION OF THE PREFERRED EMBODIENTS

The term acid gas includes CO₂ alone or in combination with H₂ S, SO₂,SO₃, CS₂, HCN, COS and the oxides and sulfur derivatives of C₁ to C₄hydrocarbons. These acid gases may be present in trace amounts within agaseous mixture or in major proportions.

The contacting of the absorbent mixture and the acid gas may take placein any suitable contacting tower. In such processes, the gaseous mixturefrom which the acid gases are to be removed may be brought into intimatecontact with the absorbing solution using conventional means, such as atower packed with, for example, ceramic rings or with bubble cap platesor sieve plates, or a bubble reactor.

In a preferred mode of practicing the invention, the absorption step isconducted by feeding the gaseous mixture into the base of the towerwhile fresh absorbing solution is fed into the top. The gaseous mixturefreed largely from acid gases emerges from the top. Preferably, thetemperature of the absorbing solution during the absorption step is inthe range from about 25° to about 200° C., and more preferably from 35°to about 150° C. Pressures may vary widely; acceptable pressures arebetween 5 and 2000 psia, preferably 100 to 1500 psia, and mostpreferably 200 to 1000 psia in the absorber. In the desorber, thepressures will range from about 5 to 100 psig. The partial pressure ofthe acid gas, e.g., CO₂ in the feed mixture will preferably be in therange from about 0.1 to about 500 psia, and more preferably in the rangefrom about 1 to about 400 psia. The contacting takes place underconditions such that the acid gas, e.g., CO₂, is absorbed by thesolution. Generally, the countercurrent contacting to remove the acidgas will last for a period of from 0.1 to 60 minutes, preferably 1 to 5minutes. During absorption, the solution is maintained in a singlephase. N-secondary butyl glycine aids in reducing foam in the contactingvessels.

The aqueous absorption solution comprising the alkaline material, theactivator system comprising the sterically hindered monosubstitutedamino acid, N-secondary butyl glycine, which is saturated or partiallysaturated with gases, such as CO₂ and H₂ S may be regenerated so that itmay be recycled back to the absorber. The regeneration should also takeplace in a single liquid phase. Therefore, the presence of the highlywater soluble amino acid provides an advantage in this part of theoverall acid gas scrubbing process. The regeneration or desorption isaccomplished by conventional means, such as pressure reduction, whichcauses the acid gases to flash off or by passing the solution into atower of similar construction to that used in the absorption step, at ornear the top of the tower, and passing an inert gas such as air ornitrogen or preferably steam up the tower. The temperature of thesolution during the regeneration step may be the same as used in theabsorption step, i.e., 25°to about 200° C., and preferably 35° to about150° C. The absorbing solution, after being cleansed of at least aportion of the acid bodies, may be recycled back to the absorbing tower.Makeup absorbent may be added as needed. The use of N-secondary butylglycine enables one to maintain a single phase regardless of the CO₂content in the acid gas.

As a typical example, during desorption, the acid gas, e.g., CO₂ -richsolution from the high pressure absorber is sent first to a flashchamber where steam and some CO₂ are flashed from solution at lowpressure. The amount of CO₂ flashed off will, in general, be about 35 to40% of the net CO₂ recovered in the flash and stripper. This isincreased somewhat, e.g., to 40 to 50%, with the high desorption ratepromoter system owing to a closer approach to equilibrium in the flash.Solution from the flash drum is then steam stripped in the packed orplate tower, stripping steam having been generated in the reboiler inthe base of the stripper. Pressure in the flash drum and stripper isusually 16 to about 100 psia, preferably 16 to about 30 psia, and thetemperature is in the range from about 25° to about 200° C., preferably35° to about 150° C., and more preferably 100° to about 140° C. Stripperand flash temperatures will, of course, depend on stripper pressure,thus at about 16 to 25 psia stripper pressures, the temperature willpreferably be about 100° to about 140° C. during desorption. Singlephase is maintained and facilitated by use of the N-secondary butylglycine promoter.

In the most preferred embodiment of the present invention, the acid gas,e.g., CO₂ is removed from a gaseous stream by means of a process whichcomprises, in sequential steps, (1) contacting the gaseous stream with asolution comprising 10 to about 40 weight percent, preferably 20 toabout 30 weight percent of potassium carbonate, an activator or promotersystem comprising 2 to about 20 weight percent, preferably 5 to about 15weight percent, more preferably 5 to about 10 weight percent of thesterically hindered monosubstituted amino acid, N-secondary butylglycine, the balance of said solution being comprised of water, saidcontacting being conducted at conditions whereby the acid gas isabsorbed in said solution, and preferably at a temperature ranging from25° to about 200° C., more preferably from 35° to about 150° C. and apressure ranging from 100 to about 1500 psig, and (2) regenerating saidsolution at conditions whereby said acid gas is desorbed from saidsolution. By practicing the present invention, one can operate theprocess above described at conditions whereby the working capacity,which is the difference in moles of acid gas absorbed in the solution atthe termination of steps (1) and (2 ) based on the moles of potassiumcarbonate originally present, is greater than obtained under the sameoperating conditions for removing acid gases from gaseous streams,wherein said same operating conditions do not include N-secondary butylglycine as the promoter. In other words, working capacity is defined asfollows: ##EQU1##

It should be noted that throughout the specification wherein workingcapacity is referred to, the term may be defined as the differencebetween CO₂ loading in solution at absorption conditions (step 1) andthe CO₂ loading in solution at regeneration conditions (step 2) eachdivided by the initial moles of potassium carbonate. The workingcapacity is equivalent to the thermodynamic cyclic capacity, that is theloading is measured at equilibrium conditions. This working capacity maybe obtained from the vapor-liquid equilibrium isotherm, that is, fromthe relation between the CO₂ pressure in the gas and the acid gas, e.g.,CO₂ loading in the solution at equilibrium at a given temperature. Tocalculate thermodynamic cyclic capacity, the following parameters mustusually be specified: (1) acid gas, e.g., CO₂ absorption pressure, (2)acid gas, e.g., CO₂ regeneration pressure, (3) temperature ofabsorption, (4) temperature of regeneration, (5) solution composition,that is weight percent N-secondary butyl glycine and the weight percentof the alkaline salt or hydroxide, for example potassium carbonate, and(6) gas composition. The skilled artisan may conveniently demonstratethe improved process which results by use of the sterically hinderedamino acid by a comparison directly with a process wherein thesterically hindered amino acid is not included in the aqueous scrubbingsolutions. For example, it will be found when comparing two similar acidgas scrubbing processes (that is similar gas composition, similarscrubbing solution composition, similar pressure and temperatureconditions) that when the sterically hindered amines are utilized thedifference between the amount of acid gas, e.g., CO₂ absorbed at the endof step 1 (absorption step) defined above and step 2 (desorption step)defined above is significantly greater. This significantly increasedworking capacity is observed even though the scrubbing solution that isbeing compared comprises an equimolar amount of a prior art aminepromoter, such as diethanolamine, 1,6-hexanediamine, etc. It has beenfound that the use of the N-secondary butyl glycine of the inventionprovides a working capacity which is at least 15% greater than theworking capacity of a scrubbing solution which does not utilize thesterically hindered amino acid. Working capacity increases of from 20 to60% may be obtained by use of the sterically hindered amino acidcompared to diethanolamine.

Besides increasing working capacity and rates of absorption anddesorption, the use of the N-secondary butyl glycine leads to lowersteam consumption during desorption.

Steam requirements are the major part of the energy cost of operating anacid gas, e.g., CO₂ scrubbing unit. Substantial reduction in energy,i.e., operating costs will be obtained by the use of the processutilizing N-secondary butyl glycine. Additional savings from new plantinvestment reduction and debottlenecking of existing plants may also beobtained by the use of N-secondary butyl glycine. The removal of acidgases such as CO₂ from gas mixtures is of major industrial importance,particularly the systems which utilize potassium carbonate activated bythe unique activator or promoter system of the present invention.

While the sterically hindered amines, as shown in U.S. Pat. No.4,112,050, provide unique benefits in their ability to improve theworking capacity in the acid scrubbing process, their efficiencydecreases in alkaline "hot pot" (hot potassium carbonate) scrubbingsystems at high temperatures and at low concentrations of the acid gasdue to phase separation. Therefore, full advantage of these highlyeffective sterically hindered amines cannot always be utilized at theseoperating conditions. The addition of an amino acid, as a cosolvent andcopromoter as shown in U.S. Pat. No. 4,094,957, solves the problem ofphase separation and enables a more complete utilization of stericallyhindered amines as the alkaline materials activator or promoter. Many ofthe disclosed amino acids when used alone, such as pipecolinic acid,while soluble in these alkaline systems, are not as effective asactivators in acid gas scrubbing processes as the other stericallyhindered amines. Subsequent tests have confirmed that most amino acidsare not as effective as N-cyclohexyl 1,3-propanediamine. Therefore, itwas not expected that N-secondary butyl glycine, as the sole promoter,would provide high working capacity and high rates of CO₂ absorption.

The absorbing solution of the present invention, as described above,will be comprised of a major proportion of two alkaline materials, e.g.,alkali metal salts or hydroxides and a minor proportion of the aminoacid activator system. The remainder of the solution will be comprisedof water and/or other commonly used additives, such as anti-foamingagents, antioxidants, corrosion inhibitors, etc. Examples of suchadditives include arsenious anhydride, selenious and tellurous acid,protides, vanadium oxides, e.g., V₂ O₃, chromates, e.g., K₂ Cr₂ O₇, etc.

The N-secondary butyl glycine compound useful in the practice of thepresent invention is either available commercially or may be prepared byvarious known procedures. N-secondary butyl glycine has the CAS RegistryNumber of 58695-42-4 and is mentioned as an intermediate in several U.S.patents, e.g., U.S. Pat. Nos. 3,894,036; 3,933,843; 3,939,174 and4,002,636, as well as the published literature (Kirino et al., Agric.Biol. Chem., 44(1), 31 (1980), but nothing is said in these disclosuresabout the synthesis of this amino acid or its use as a carbonatepromoter in acid gas scrubbing processes.

A preferred method for preparing N-secondary butyl glycine comprisesreacting glycine under reductive conditions with methyl ethyl ketone inthe presence of a hydrogenation catalyst. This reaction produces thesterically hindered monosubstituted amino acid N-secondary butyl glycinein nearly quantitative yields. This process is more fully described andclaimed in U.S. Ser. No. 321,058, filed concurrently herewith, entitled,"Amino Acids and Process for Preparing the Same" (G. Sartori and W.Thaler), the disclosure of which is incorporated herein by reference.

The invention is illustrated further by the following examples which,however, are not to be taken as limiting in any respect. All parts andpercentages, unless expressly stated to be otherwise, are by weight.

EXAMPLE 1 "Hot Pot" Acid Gas Treating Process

The reaction apparatus consists of an absorber and a desorber as shownin FIG. 1 of U.S. Pat. No. 4,112,050, incorporated herein by reference.The absorber is a vessel having capacity of 2.5 liters and a diameter of10 cm, equipped with a heating jacket and a stirrer. A pump removesliquid from the bottom of the reactor and feeds it back to above theliquid level through a stainless-steel sparger. Nitrogen and CO₂ can befed to the bottom of the cell through a sparger.

The desorber is a 1-liter reactor, equipped with teflon blade stirrer,gas sparger, reflux condenser and thermometer.

The following reagents are charged into a 2-liter Erlenmeyer flask:

92 g of N-secondary butyl glycine

225 g of K₂ CO₃

433 g of water

When all solid has dissolved, the mixture is put into the absorber andbrought to 80° C. The apparatus is closed and evacuated until the liquidbegins to boil. At this point, CO₂ is admitted into the absorber.Thirty-two (32) liters of CO₂ is absorbed.

The rich solution so obtained is transferred to the desorber and boiledfor one hour, during which time 28 liters of CO₂ is desorbed. Theregenerated solution so obtained is put into the absorber and cooled to80° C. The apparatus is closed and evacuated until the liquid begins toboil. At this point CO₂ is admitted. 29.6 liters of CO₂ is absorbed, ofwhich 13 liters is absorbed in the first minute.

EXAMPLE 2

The procedure of Example 1 is repeated after replacing N-secondary butylglycine with an equimolar amount of other amino compounds, includingstructurally related sterically hindered amines and amino acids, andcorrecting the amount of water in order to have a total initial weightof 750 g. The results of these tests are shown in Table I.

                  TABLE I                                                         ______________________________________                                        CO.sub.2 SCRUBBING BY AMINO ACID                                              PROMOTED K.sub.2 CO.sub.3 SOLUTIONS                                                           Liters of CO.sub.2 Absorbed                                                   Into Regenerated Solution                                     Amino acid        Total   First Minute                                        ______________________________________                                        N--sec. butyl glycine                                                                           29.6    13                                                  N--cyclohexyl glycine                                                                           30.8    14                                                  N--isopropyl glycine                                                                            27.5    10                                                  N--(2-amyl)-glycine                                                                             26.5    17                                                  N--sec. butyl alanine                                                                           15      3                                                   N--isopropyl alanine                                                                            25.2    4                                                   Pipecolinic Acid  22.5    8                                                   ______________________________________                                    

The data in Table I show that N-secondary butyl glycine and N-cyclohexylglycine are superior promoters to the other tested sterically hinderedamines and amino acids of similar structure.

In view of the data shown in Table I, additional tests were conductedwith N-secondary butyl glycine and N-cyclohexyl glycine to ascertaintheir suitability in large scale acid gas scrubbing operations.

EXAMPLE 3 (a) Aging Studies in CO₂ Scrubbing Apparatus

The following experiments are carried out to ascertain the stability ofN-secondary butyl glycine and N-cyclohexyl glycine underaccelerated-simulated acid gas treating conditions.

The following reagents are charged into a stainless-steel bomb:

121 g of N-secondary butyl glycine

433 g of KHCO₃

540 g of H₂ O

The bomb is put into an oven and heated at 120° C. for 1000 hours. Thenthe content is discharged into a 2 liter flask and refluxed for severalhours.

750 g is taken and subjected to an absorption-desorption-reabsorptioncycle as described in Example 1. 27.9 liters of CO₂ is absorbed into theregenerated solution, 10 liters being absorbed in the first minute.

Comparison of this result with that obtained with the fresh solution,described in Example 1, shows that the aging process does not lead to asignificant loss of activity.

If the aging experiment is carried out after replacing N-secondary butylglycine with the equivalent amount of N-cyclohexyl glycine, 145 g, andreducing the water to 516 g in order to have the same total weight, aconsiderable amount of solid, identified as1,4-bis-cyclohexyl-2,5-diketopiperazine is formed. An attempt to carryout an absorption-desorption cycle causes plugging of the unit.

(b) Aging Under CO₂ and H₂ S

The following reagents are charged into a stainless-steel bomb:

121 g of N-secondary butyl glycine

24 g of K₂ S

390 g of KHCO₃

544 g of water

The bomb is put into an oven and heated at 120° C. for 1000 hours. Thenthe content is discharged into a 2 liter flask and refluxed for severalhours.

765 g is taken and subjected to an absorption-desorption-reabsorptioncycle as described in Example 1. 28.9 liters of CO₂ is absorbed into theregenerated solution, 10 g being absorbed in the first minute.Comparison of this result with that obtained with the fresh solution,described in Example 1, shows that the aging process leads to only aslight loss of activity.

The excellent stability under the aging conditions shown above for theN-secondary butyl glycine coupled with its good performance as apromoter, demonstrates the desirability of using this amino acid ratherthan N-cyclohexylglycine.

EXAMPLE 4

Vapor-liquid equilibrium measurements were carried out to confirm thatN-secondary butyl glycine leads to a broadening of cyclic capacity (asdefined in U.S. Pat. No. 4,112,050, incorporated herein by reference) ascompared to diethanolamine owing to a shift in the equilibrium position.

The vapor-liquid equilibrium measurements are made by first preparingthe following solution:

    ______________________________________                                        73.6       g of N--secondary butyl glycine                                    150.0      g of K.sub.2 CO.sub.3                                              376.4      g H.sub.2 O                                                        600.0      g                                                                  ______________________________________                                    

The solution is charged into a one-liter autoclave, equipped withstirrer, condenser, inlet and outlet tube for gases and liquid-samplingline. The autoclave is brought to 250° F. while blowing through thesolution a mixture of 20 mol % CO₂ and 80 mol % He. The rate at whichthe gaseous mixture is fed is 0.2 liter/min.

The pressure is 300 psig. When the outgoing gas has the same compositionas the entering gas, equilibrium has been reached. A sample of liquid istaken and analyzed. The CO₂ content is 13.0 wt. %, the K content is13.6%, from which a carbonation ratio of 1.70 is calculated. Bycarbonation ratio, it is meant the molar ratio of CO₂ absorbed toinitial K₂ CO₃.

The operation is repeated several times, changing the composition of thegas and the total pressure. If the partial pressures of CO₂ are plottedagainst the carbonation ratios, the curve shown in FIG. 1 is obtained.

Using the same procedure, the vapor-liquid equilibrium curve isdetermined, using diethanolamine in an amount equivalent to the totalamino acid amount used above. The resulting vapor-liquid equilibriumcurve is also shown in FIG. 1.

In the interval of P_(CO).sbsb.2 studied, i.e., from 0.08 to 300 psia,N-secondary butyl glycine leads to a larger cyclic capacity thandiethanolamine.

While the invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodification, and this application is intended to cover any variations,uses, or adaptations of the invention following, in general, theprinciples of the invention and including such departures from thepresent disclosure as come within known or customary practice in the artto which the invention pertains and as may be applied to the essentialfeatures hereinbefore set forth, and as fall within the scope of theinvention.

What is claimed is:
 1. A process for the removal of CO₂ from a gaseousstream containing CO₂ which comprises contacting said gaseous stream (1)in an absorption step with an aqueous absorbing solution comprising (a)a basic alkali metal salt or hydroxide selected from the groupconsisting of alkali metal bicarbonates, carbonates, hydroxides,borates, phosphates, and their mixtures, and (b) an activator orpromoter system for said basic alkali metal salt or hydroxide comprisingN-secondary butyl glycine; and (2) in a desorption and regenerationstep, desorbing at least a portion of the absorbed CO₂ from saidabsorbing solution.
 2. The process of claim 1 wherein the basic alkalimetal salt or hydroxide is potassium carbonate.
 3. The process of claim1 wherein the aqueous solution contains 10 to about 40% by weight ofsaid basic alkali metal salt or hydroxide.
 4. The process of claim 1wherein the aqueous solution contains 2 to about 20% by weight of saidN-secondary butyl glycine.
 5. The process of claim 4 wherein the aqueoussolution contains 5 to about 15% by weight of said N-secondary butylglycine.
 6. The process of claim 4 wherein the absorbing solution fromthe regenerating step is recycled for reuse in the absorption step. 7.The process of claim 1, 2, 3, 4, 5 or 6 wherein the temperature of theabsorbing solution during the absoption step is in the range from about25° to about 200° C., the pressure in the absorber ranges from about 5to about 200 psia and the partial pressure of the acid gas components inthe feed stream ranges from about 1.0 to about 500 psia, and wherein thetemperature of the absorbing solution during the regeneration stepranges from about 25° to about 200° C., and at pressures ranging fromabout 16 to about 100 psia.
 8. The process of claim 1, 2, 3, 4, 5, or 6wherein the absorbing solution additionally includes additives selectedfrom the group consisting of antifoaming agents, antioxidants andcorrosion inhibitors.
 9. A process for the removal of CO₂ from a gaseousstream containing CO₂ which comprises, in sequential steps, (1)contacting the gaseous stream with an absorbing solution comprising (a)from about 20 to about 30% by weight of potassium carbonate, and (b) anactivator or promoter system for the potassium carbonate, comprisingfrom about 5 to about 15% by weight of N-secondary butyl glycine as thesole promoter, (c) the balance of the solution comprising water andadditives selected from the group consisting of antifoaming agents,antioxidants and corrosion inhibitors, wherein said contacting isconducted at conditions whereby CO₂ is absorbed in said absorbingsolution and the temperature of the absorbing solution is in the rangefrom about 35° to about 150° C., and the pressure in the absorber is inthe range from about 100 to about 1500 psig; and (2) regenerating saidabsorbing solution at conditions whereby CO₂ is desorbed from saidabsorbing solution, wherein the regeneration takes place at temperaturesranging from about 35° to about 150° C. and at pressures ranging fromabout 5 to about 100 psig.
 10. The process of claim 9 wherein theabsorbing solution from the regeneration step is recycled for reuse inthe absorption step.
 11. An aqueous acid gas scrubbing compositioncomprising: (a) 10 to about 40% by weight of an alkali metal salt orhydroxide, (b) 2 to about 20% by weight of the sterically hinderedmonosubstituted amino acid, N-secondary butyl glycine and (c) thebalance, water.
 12. The composition of claim 11 wherein said alkalimetal salt or hydroxide is potassium carbonate.
 13. An aqueous acid gasscrubbing composition comprising: (a) 20 to 30% by weight of potassiumcarbonate, (b) 5 to about 15% by weight of N-secondary butyl glycine,and (c) the balance, water.
 14. The composition of claims 11, 12 or 13wherein the composition additionally includes antifoaming agents,antioxidants and corrosion inhibitors.